1. Field of the Invention
The present invention relates to a color image reading device that reads information on an original copy to obtain the information as electric signals.
2. Description of the Related Art
A typical color image reading device is disclosed by Japanese Patent Application Laid-open No. Sho 54-81715, which successively switches light sources of three colors (red/green/blue) to irradiate a color original copy, and which successively reads the information on the original copy with a monochrome image sensor. Recent development can permit an LED to be used as each of three color light sources to thereby make the device compact.
FIG. 2 is a plane view showing an example of a monochrome image sensor that is used in a conventional color image reading device. FIG. 3 shows a timing chart of the conventional color image reading device.
In FIG. 2, image sensor ICs 1-1 to 1-9 which have the same construction are linearly mounted on a circuit board 7. Each image sensor IC has a start signal input terminal 3, a start signal output terminal 5, and an image signal output terminal 4, which are electrically connected to a wiring pattern 12 on the circuit board 7 through bonding wires 6. Each image sensor IC is provided with a light receiving element array 2. Although not shown in the drawings, clock pulses and electric power are supplied to the image sensor ICs through the bonding wires 6.
The start signal input terminal 3 of the image sensor IC 1-1 is connected to a start signal input terminal 8 of the circuit board 7. The start signal input terminals 3 of the image sensor IC 1-2 and other succeeding image sensor ICs are respectively connected to the start-signal output terminals 5 of the one-preceding image sensor ICs.
The image sensor IC initiates the output through the image signal output terminal 4 when the image sensor IC receives a start pulse through the start signal input terminal 3. The image sensor IC successively outputs the outputs of the light receiving elements in each-bit basis synchronously with the clock pulses or the like. At near timing to output the last bit of the light receiving element, the image sensor IC outputs a start pulse through the start signal output terminal 5. This start pulse is used as a start pulse for the succeeding image sensor IC. The outputs of the light receiving elements of all image sensor ICs are read out successively in this manner. The image signal output terminals 4 of all image sensor ICs are connected to an image signal output terminal 13 of the circuit board 7, so that electric signals are externally output to be subjected to processing such as A/D conversion.
The image sensor IC initializes a light receiving element immediately after the output from that light receiving element is ended, and thereafter starts charging the carriers generated by the next photo-electric conversion. Therefore, a conventional color image sensor, in which the image sensor as shown in FIG. 2 is used to read a color image, performs the irradiation by the three color light sources and the output of those colors in such a timing as shown in FIG. 3. That is, immediately after the output in each bit is ended, the charge for that bit is started, so that the image signal cannot be output during the light source is ON. For this reason, the sum of the time period for the irradiation of three colors and the time period for the output of three color image signals is required to read information of three colors on one line. Thus, there arises a problem in that the reading time per one line is long.
The detailed description will be given with reference to FIG. 3. Immediately after the RED light source is set to be OFF, the start pulse is entered through the start signal input terminal 8 of the circuit board 7 to the image sensor IC 1-1, and thereafter the image signal outputs of RED component from the light receiving element are started. After the outputs of all image sensor ICs are ended, the Green light source is set to be ON, so that all light receiving elements charge carriers generated due to the reflected light from the original copy irradiated by GREEN light source. Here, if the GREEN light source is set to be ON before the image signal outputs of RED components from all image sensor ICs are ended, the outputs which have not yet been output from the image sensor ICs are inevitably mixed with the component of the GREEN reflected light. Therefore, this is not acceptable case. When the start pulse is entered through the start signal input terminal 8 of the circuit board 7 to the image sensor IC 1-1 immediately after the GREEN light source is set to be OFF, the image signal output of GREEN component from the light receiving element is started. The similar procedure is carried out with respect to BLUE. Thus, the reading time period T for one line is expressed by the following formula:
T=TP(R)+TP(G)+TP(B)+3xc3x97Cxc3x97Bxc3x97TY
Here, TP(R), TP(G) and TP(B) respectively indicate the time period when the RED light source is ON, the time period when the GREEN light source is ON and the time period when the BLUE light source is ON. The character C designates the number of image sensor ICs; B, the number of light receiving elements in one image sensor IC; and TY, the reading time period per one bit.
For example, in case of an image sensor having a width of A4 sheet size and 300 DPI resolution, if the number of light receiving elements is 288 (B=288), then the number of image sensor ICs is 9 (C=9), and further if the reading time period per one bit is 500 nsec (TY=500 nsec), then the image signal output time period for one line is 3.888 msec (3xc3x97Cxc3x97Bxc3x97TY=3.888 msec).
Assuming that each of TP(R) TP(G) and TP(B) is 1 msec (TP(R)=TP(G)=TP (B)=1 msec), the time period when light sources are ON is 3 msec (TP(R)+TP(G)+TP(B)=3 msec).
Therefore, the total reading time period T is about 6.888 msec (T=6.888 msec), so that the image signal output time period occupies 56% of the reading time period for one line.
Such an IC is available, in which memory means each having capacity corresponding to each bit are provided within the IC so that the output of each bit is temporality input into the memory means and thereafter read out from the memory means in order. In this case, since the light can be emitted from the light source of the next color during the output time period, the reading time period per one line is substantially equal to the total irradiation time period of three color light sources. However, there arises a problem in that the provision of the memory means within the IC results in the increase in the cost of the image sensor IC.
The irradiation time period of the light source depends on luminance of the light source and the S/N ratio of the image sensor, and thus can not be readily shortened. Further, the output time period depends on the activation speed of the IC, and thus it is hardly shortened.
In order to solve these problems associated with the conventional art, the present invention provides a color image reading device that can shorten the reading time period, and can be manufactured with low cost.
In order to solve the above-mentioned problems, the present invention provides a color image reading device having the following arrangement.
In a color image reading device having three or more color light sources and an image sensor mounting a plurality of image sensor ICs linearly thereon, the image sensor ICs are divided into two or more blocks, and image signal outputs of all blocks are read out simultaneously.
In a case where the image sensor ICs are divided into N blocks so that image output signals of all blocks are read out simultaneously, the image signal output time period for one line can be reduced to be about 1/N of that in the conventional device. Therefore, a color image reading device featured by simple construction and less reading time period can be obtained.